Takes a function and returns the names of the parameters for that function.

getParamNames.js

/**
 * Takes a function and returns the names of its parameters.
 * @see https://gist.github.com/westc/b9c62d86efc015d1a444#file-getparamnames-js
 * @param {Function} fn
 *   The function for which to get the parameter names.
 * @returns {string[] | undefined}
 *   If `fn` is really a function and when using its `toString()` the parameters
 *   could be seen as a string then this will be an array of all of the
 *   parameters in this function.  Otherwise `undefined` will be returned.
 */
function getParamNames(fn) {
  var params = (fn+'').replace(/\/\*[\s\S]*?\*\/|\/\/.*?[\r\n]/g, '').match(/\(([^]*?)\)/);
  return ('function' === typeof fn && params) ? params[1].trim().split(/\s*,\s*/) : undefined;
}